The present disclosure relates to a spin transfer torque magnetic memory device, and in particular, to a magnetic memory device, in which magnetization reversal of a free magnetic layer of a magnetic tunnel junction of a unit cell is used to store or read data.
A ferromagnetic material is a material exhibiting a spontaneous magnetization property, even when there is no external magnetic field. A tunnel junction unit cell includes a magnetic tunnel junction (MTJ) structure, in which two ferromagnetic layers and an insulating layer therebetween (e.g., a first ferromagnetic layer, an insulating layer, and a second ferromagnetic layer) are provided.
Electric resistance of the MTJ structure is changed depending on relative orientation between the magnetization directions of the first and second ferromagnetic layers. This is called a tunneling magnetoresistance effect and results from a difference between the tunneling of up- and down-spin electrons through the insulating layer. Due to the tunneling magnetoresistance effect, the relative orientation between the magnetization directions of the first and second ferromagnetic layers may be used to control a current flowing through the tunnel junction unit cell.
Meanwhile, according to the Newton's third law, namely the law of action and reaction, if a flow of current can be controlled by the magnetization direction, it is also possible to control a magnetization direction of the magnetic layer by applying a current by the reaction. In the case that there is a current flowing through the MTJ structure in a thickness direction perpendicular to the tunnel junction unit cell, a spin-polarized current caused by the first or pinned magnetic layer may pass through the second or free magnetic layer, and spin angular momentum of the spin-polarized current may be transferred to magnetization of the second magnetic layer. This process is called ‘spin transfer torque’, and a magnetization direction of the free magnetic layer may be reversed or changed by using the spin transfer torque.
FIG. 1 is a sectional view illustrating a structure of a conventional spin transfer torque magnetic memory device.
Referring to FIG. 1, a conventional magnetic memory device includes a tunnel junction unit cell having ferromagnetic layers, each of which has a magnetization perpendicular to a surface thereof, and an insulating layer, which is interposed between the ferromagnetic layers.
That is, the magnetic memory device includes a first electrode, a tunnel junction unit cell, and a second electrode, and the tunnel junction unit cell includes a first or pinned magnetic layer 101, an insulating layer 102, and a second or free magnetic layer 103 whose magnetization direction can be changed by an electric current. Here, the second magnetic layer is connected to the electrode, and in this case, a current flowing in a direction perpendicular to a surface of a layer can be used to induce magnetization reversal. Depending on relative orientation between magnetization directions of the pinned and free magnetic layers, electrical signals can have two different resistance states of high resistance and low resistance, and this may be used to realize a magnetic memory device, in which data of “0” or “1” are stored.
Meanwhile, in the case that an external magnetic field, not electric current, is used to control the magnetization of the free magnetic layer, the smaller the device, the more difficult it is to avoid an issue of half-selected cell, and thus, it is difficult to realize a highly integrated magnetic memory device. By contrast, in the case that the spin transfer torque produced by the electric current is used, it is easy to reverse magnetization of a selected cell, regardless of a size of the device.
In the afore-described spin transfer torque magnetic memory device using electric current, a magnitude of a spin transfer torque produced in the free magnetic layer may be determined by a current density applied thereto. This means that there is a critical current density for magnetization reversal of the free magnetic layer.
In the case that both of the pinned and free magnetic layers are formed of a material with perpendicular magnetic anisotropy (PMA), the critical current density JC may be given by the following equation 1.
                              J          C                =                                                            2                ⁢                                                                  ⁢                e                            ℏ                        ⁢                                          α                ⁢                                                                  ⁢                                  M                  S                                ⁢                d                            η                        ⁢                          (                                                H                                      K                    ⊥                                                  -                                                      N                    d                                    ⁢                                      M                    S                                                              )                                =                                                    2                ⁢                                                                  ⁢                e                            ℏ                        ⁢                                          α                ⁢                                                                  ⁢                                  M                  S                                ⁢                d                            η                        ⁢                          (                              H                                  K                  ,                  eff                                            )                                                          [                  Equation          ⁢                                          ⁢          1                ]            
where α is the Gilbert damping constant,  (=1.05×10−34 J·s) is equal to the Planck constant divided by 2π, e (=1.6×10−19 C) is the electric charge of an electron, η is a spin polarization efficiency constant which is determined by a material and a structure of a device and has a value between 0 and 1, MS is a saturation magnetization of a magnetic material, d is a thickness of a free magnetic layer, HK⊥=(2K⊥/MS) is a PMA field of the free magnetic layer, K⊥ is a PMA energy density of the free magnetic layer, HK,eff is an effective anisotropy field, in a perpendicular direction, of the free magnetic layer and is defined as HK,eff=(HK⊥−NdMS), and Nd is an effective demagnetization field constant in a perpendicular direction and has a value between 0 to 4π in CGS units, depending on a shape of the free magnetic layer.
If a size of a magnetic junction unit cell included in a highly-integrated memory device decreases, there may be a superparamagnetic limit, at which a direction of magnetization written is arbitrarily changed due to heat energy at room temperature. This may lead to a problem such as an unintended erase of written magnetic information. Time τ, in which magnetization direction is averagely maintained against heat energy, is given by the following equation 2.
                    τ        =                                            τ              0                        ⁢                          exp              ⁡                              (                                                                            K                      eff                                        ⁢                    V                                                                              k                      B                                        ⁢                    T                                                  )                                              =                                    τ              0                        ⁢                          exp              ⁡                              (                                                                            H                                              K                        eff                                                              ⁢                                          M                      s                                        ⁢                    V                                                        2                    ⁢                                                                                  ⁢                                          k                      B                                        ⁢                    T                                                  )                                                                        [                  Equation          ⁢                                          ⁢          2                ]            
where τ is a reciprocal of attempt frequency and has a value of about 1 ns, Keff is an effective magnetic anisotropy energy density (=HK,effMS/2) of the free magnetic layer, V is a volume of the free magnetic layer, kB is the Boltzmann's constant (=1.381×10−16 erg/K), and T is a Kelvin temperature.
In addition, KeffV/KBT is defined as thermal stability Δ of a magnetic memory device. The condition of Δ>50 should be generally satisfied for the purpose of commercializing it as a nonvolatile memory device. If a volume V of the free magnetic layer is reduced to realize a highly-integrated device, Keff should be increased to meet the condition of Δ>50.
However, in the case that the effective magnetic anisotropy energy density Keff of the free magnetic layer is increased, there may be a problem, such as an increase of the critical current density JC.
In the case where a spin transfer torque is used to induce the magnetization reversal in the conventional structure illustrated in FIG. 1, it is very difficult to obtain a sufficiently high thermal stability Δ and a sufficiently low critical current density JC that are suitable for commercialization, because both of the thermal stability Δ and the critical current density JC are proportional to Keff.
In addition, an amount of a current, which can be supplied through a current-applying device (e.g., a transistor) for supply a current to an MTJ structure, may generally be proportional to a size of the current-applying device. This means that, in order to supply a current density greater than the critical current density JC, the size of the device should be maintained to be larger than a proper size. Accordingly, the size of the current-supplying part for supplying a current greater than JC may set a limit on an integration density of a magnetic memory device.
Also, in the case where, in a conventional structure, a current flows through an MTJ structure, the larger the thickness of the insulating layer, the larger a difference between an up-spin and a down-spin of a tunneling electron, and this may lead to an increase in tunneling magnetoresistance. However, the larger the thickness of the insulating layer, the smaller an amount of a tunneling current itself, and thus, it may be difficult to effectively transfer a spin transfer torque for the magnetization reversal to the magnetization of the free magnetic layer. In other words, if the insulating layer is thickened, a value of tunneling magnetoresistance may be increased to make it possible to read a magnetization state at a very fast speed, but it may lead to a reduction in current density. Accordingly, it may be difficult to realize a magnetic memory device satisfying both requirements for the tunneling magnetoresistance and the current density.
FIG. 2 is a sectional view illustrating a conventional magnetic memory device including tunnel junction unit cells, each of which has a conventional MTJ structure using a spin-orbit spin transfer torque and an external magnetic field.
FIG. 2 illustrates a magnetic memory device using a spin-orbit spin transfer torque, which was invented and filed by one of the inventors of the present application and was issued as Korea Patent No. 10-1266791, and the magnetic memory device of FIG. 2 may be used to overcome the afore-described problems. It may be possible to overcome two problems in the conventional MTJ structure, in which a spin transfer torque by a current flowing in a perpendicular direction is used to induce magnetization reversal of a free magnetic layer. In other words, it may be possible to overcome the problems of (i) since the critical current density and the thermal stability are proportional to the same material parameter (i.e., Keff or the effective magnetic anisotropy energy density of the free magnetic layer), it is difficult to meet both requirements for commercialization (i.e., the sufficiently low critical current density and the sufficiently high thermal stability) and (ii) if an insulating layer of the MTJ structure is thickened, the tunneling magnetoresistance is increased to more quickly read a magnetization state, but the current density is decreased to make it difficult to change the magnetization state.
Furthermore, in order to realize a highly-integrated magnetic memory device, a conductive line having a non-magnetic property may be provided adjacent to a free magnetic layer. In such a magnetic memory device, by using a spin Hall effect or Rashba effect to be caused by an in-plane current flowing through the conductive line, it may be possible to induce the magnetization reversal of the free magnetic layer through a spin-orbit spin transfer torque. In addition, the magnetic memory device may be configured in such a way that the magnetization reversal of each cell can be selectively realized using a voltage to be selectively applied to each MTJ memory cell.
However, in order to operate a conventional magnetic memory using spin-orbit spin transfer torque, an additional in-plane magnetic field is necessarily required. This is because an antidamping torque τA, an antidamping component of the spin-orbit spin transfer torque inducing the switching of the perpendicular magnetization, is given by the following equation.τA=γcJ{circumflex over (m)}×({circumflex over (m)}×ŷ)  [Equation 3]
where γ is a gyromagnetic ratio, CJ is a magnitude of an antidamping component of a spin-orbit spin torque, which is expressed in the unit of magnetic field and is proportional to a current density, {circumflex over (m)} is a unit vector of a magnetization direction of a free magnetic layer, and ŷ is a unit vector of a direction which is perpendicular to both of a current direction {circumflex over (x)} and a perpendicular direction {circumflex over (z)} of the MTJ.
In the case where a sufficiently high current density is applied, the magnetization direction {circumflex over (m)} is changed until τA reaches zero, and thus, {circumflex over (m)} is aligned to +ŷ or −ŷ direction, depending on a sign of a current flowing in {circumflex over (x)} direction. In other words, when the magnetization is aligned to a perpendicular direction (+{circumflex over (z)} or −{circumflex over (z)}) before applying the current density, and then, if a spin-orbit spin transfer torque is applied thereto, the magnetization direction is aligned to +ŷ or −ŷ direction, depending on a sign of the current.
Next, if the current is turned off in this state, due to the PMA, it may be aligned to +{circumflex over (z)} or −{circumflex over (z)} direction with probability of half (½). That is, in the case where only a spin-orbit spin transfer torque is applied, it is hard to selectively change a magnetization direction after a switching operation. However, in order to apply a tunnel junction unit cell using the spin-orbit spin transfer torque to a memory device, it is necessary to selectively change a magnetization direction after a switching operation.
In the case where, to selectively change a magnetization direction after a switching operation, an in-plane magnetic field is applied together with a spin-orbit spin transfer torque, a magnetization direction may have a perpendicular component that is not parallel to the ŷ direction. Thus, if both of the spin-orbit spin transfer torque and the in-plane magnetic field are applied, it may be possible to realize a selective switching (e.g., see I. M. Miron et al., Nature 476, 189 (2011)).
However, in order to produce an additional in-plane magnetic field, it is necessary to provide an additional circuit for applying a uniform magnetic field throughout a magnetic memory array, to provide an additional conductive line for generating a magnetic field using a current, or to provide an additional horizontal magnetic layer for generating an in-plane magnetic field in a portion of the MTJ structure. This may lead to problems, such as an additional power loss, an increase in total thickness of the structure, and an increase in fabrication cost.